Organic synthetic-dyes are extensively used in various industries such as the textile, leather tanning, paper production, food technology, agricultural research, light-harvesting arrays, photo-electrochemical cells, and hair-coloring. Due to their large-scale production, extensive use, and subsequent discharge of colored waste-waters, the toxic and non-biodegradable organic synthetic-dyes cause considerable environmental pollution and health-risk factors. Moreover, they also affect the sunlight penetration and oxygen solubility in the water-bodies, which in turn affect the under-water photosynthesis activity and life-sustainability. Moreover, due to their strong color even at lower concentrations, the organic synthetic-dyes generate serious aesthetic issues in the waste-water disposal. On the other hand, the toxic soluble heavy metal-cations cause serious problems to the ecosystem due to the serious health problems as a result of their accumulation in living tissues through the food-chain. Therefore, the removal of both highly stable organic synthetic-dyes and heavy metal-cations from the aqueous solutions and industry effluents is of a prime importance. Reference may be made to (V. K. Gupta, Suhas, “Application of Low-Cost Adsorbents for Dye Removal—A Review”, Journal of Environmental Management 2009, 90, 2313-2342) wherein, the investigations conducted in the past using the different adsorbents such as orange peel, rice husk, coconut shell, carbon black, zeolites, carbon nanotubes, and flyash have been summarized. Typically, flyash (solid and hollow, also known as cenospheres) which is a waste by-product of thermal power plants, comprising the mixture of different metal-oxides such as silica (SiO2, 50-85 wt. %), alumina (Al2O3, 5-20 wt. %), iron oxide (Fe2O3, 5-15 wt. %), and trace amount of oxides of other elements such as calcium, titanium, magnesium, and toxic heavy metals such as arsenic, lead, and cobalt, has been environmentally hazardous and pose major disposal and recycling problems worldwide. Reference may be made to (H. Liu, “Method to Produce Durable Non-Vitrified Fly Ash Bricks and Blocks”, U.S. Pat. No. 7,998,268; B. R. Reddy, K. M. Ravi, “Methods of Formulating a Cement Composition”, U.S. Pat. No. 7,913,757; R. L. Hill, C. R. Jolicoeur, R. Carmel, M. Page, I. Spiratos, T. C. To, “Sacrificial Agents for Fly Ash Concrete”, U.S. Pat. No. 7,901,505) wherein, flyash has been traditionally used for landfill, manufacturing constructional materials such as cement, concrete, and bricks. Reference may be made to (K. Vasanth Kumar, V. Ramamurthi, S. Srinivasan, “Modeling the Mechanism Involved During the Sorption of Methylene Blue onto Flyash”, Journal of Colloid and Interface Science 2005, 284, 14-21; M. Matheswaran, T. Karunanithi, “Adsorption of Chrysiodine R by using Fly Ash in Batch Process”; Journal of Hazardous Materials 2007, 145, 154-161; D. Mohan, K. P. Singh, G. Singh, K. Kumar, “Removal of Dyes from Wastewater Using Flyash a Low-Cost Adsorbent”, Industrial and Engineering Chemistry Research 2002, 41, 3688-3695; S. Kara, C. Aydiner, E. Demirbas, M. Kobya, N. Dizge, “Modeling the Effects of Adsorbent Dose and Particle Size on the Adsorption of Reactive Textile Dyes by Fly Ash”, Desalination 2007, 212, 282-293) wherein, new industrial applications of flyash such as conductive/non-conductive filler in polymers, binder for agglomerating reactive mine tailings, manufacturing zeolites and microfiltration membranes, and adsorption of oil from aqueous solutions have been demonstrated. Flyash has also been utilized for the adsorption of different organic synthetic-dyes including remazol red RB 133, remazol blue, rifacion yellow HED, chrysoidine R, crystal violet, Rhodamine B, C.I. reactive black, 2-picoline, and acid red (AR1) from the aqueous solutions. Reference may be made to (M. Visa, C. Bogatu, A. Duta, “Simultaneous Adsorption of Dyes and Heavy Metals from Multicomponent Solutions using Fly Ash”, Applied Surface Science 2010, 256, 5486-5491; S. Wang, M. Soudi, L. Li, Z. H. Zhu, “Coal Ash Conversion into Effective Adsorbents for Removal of Metals and Dyes from Wastewater”, Journal of Hazardous Materials 2006, B133, 243-252; K. Ojha, N. C. Pradhan, A. N. Samanta, “Zeolite from Fly Ash: Synthesis and Characterization”, Bulletin of Materials Science 2004, 27, 555-563) wherein, flyash has been utilized for the adsorption of heavy metal-cations such as Sn2+/Sn4+, Fe2+/Fe3+, Pb2+, Zn2+, Cu2+, Mn2+, Ti4+, and others from aqueous solutions. The major advantage of using flyash for these applications is that it can be separated from the treated aqueous solutions via gravity settling. However, the major drawbacks of prior art-1 are as follows.                (1) Flyash has very low specific surface-area, and as a result, exhibits very low capacity for adsorbing organic synthetic-dyes and heavy metal-cations on its surface.        (2) Novel techniques to increase the specific surface-area of flyash, without affecting its typical spherical morphology and yet increasing its capacity for adsorbing organic synthetic-dyes and heavy metal-cations, are unknown.        (3) Adsorption of organic synthetic-dyes and heavy metal-cations on the surface of flyash generates large amount of sludge which creates further handling, disposal, and recycling issues which are at present not addressed satisfactorily.        (4) Novel value-added products based on the innovative surface-modifications of flyash for the potential applications, typically the removal of organic synthetic-dyes from the aqueous solutions, are currently lacking.        (5) Novel approaches for recycling the flyash as a catalyst in the dye-removal application, by decomposing the previously adsorbed-dye from its surface, are currently lacking.        
Hence, it is vital to develop innovative approaches to enhance the specific surface-area of flyash to increase its capacity for the adsorption of organic synthetic-dyes and heavy metal-cations. Innovative approaches are also needed to be developed to decompose the previously adsorbed-dye from the surface of flyash to recycle it for the next-cycles of dye-adsorption as a catalyst Reference may be made to (S. Shukla, S. Seal, J. Akesson, R. Oder, R. Carter, K. Scammon, “Study of Mechanism of Electroless Copper Coating of Flyash Cenosphere Particles”, Applied Surface Science 2001, 181, 35-50; S. Shukla, S. Seal, Z. Rahaman, K. Scammon, “Electroless Copper Coating of Cenospheres using Silver Nitrate Activator”, Materials Letters 2002, 57, 151-156; S. Shukla, K. G. K. Warder, K. V. Baiju, T. Shijitha, “Novel Surface-Modifications for Flyash and Industrial Applications Thereof”, U.S. patent application Ser. No. 13/612,363 (Filed on 12 Sep. 2012), PCT Application No. PCT/IN2010/000735 (Filed on 11 Nov. 2010)) wherein, as far as flyash particles with the surface-adsorbed heavy metal-cations are concerned, the Sn2+ cations adsorbed on the surface of flyash particles have been reported to be useful as sensitizer in an electroless metal (Cu/Ag)-coating of flyash particles using the Sn—Pd catalyst system. However, the major drawbacks of prior art-2 are as follows.                (6) Flyash particles with the surface-adsorbed Sn2+ cations do not find other novel potential industrial applications.        (7) Flyash particles with the surface-adsorbed metal-cations, other than Sn2+, are not suitable for the electroless metal-coating application.        (8) Flyash particles with the surface-adsorbed metal-cations, other than Sn2+, have not been utilized for other potential applications.        
Hence, the new potential industrial applications are required to be invented for improving the handling, disposal, and recycling issues of flyash with the surface-adsorbed heavy metal-cations. Reference may be made to (T. Kasuga, H. Masayoshi, “Crystalline Titania and Process for Producing the Same”, U.S. Pat. No. 6,027,775; T. Kasuga, H. Masayoshi, “Crystalline Titania having Nanotube Crystal Shape and Process for Producing the Same”; U.S. Pat. No. 6,537,517; N. Harsha, K. R. Ranya, S. Shukla, S. Biju, M. L. P. Reddy, K. G. K. Warder; “Effect of Silver and Palladium on Dye-Removal Characteristics of Anatase-Titania Nanotubes”, Journal of Nanoscience and Nanotechnology 2011, 11, 2440-2449; N. Harsha, K. R. Ranya, K. B. Babitha, S. Shukla, S. Biju, M. L. P. Reddy, K. G. K. Warder, Hydrothermal Processing of Hydrogen Titanate/Anatase-Titania Nanotubes and Their Application as Strong Dye-Adsorbents”, Journal of Nanoscience and Nanotechnology 2011, 11, 1175-1187; P. Hareesh, K. B. Babitha, S. Shukla, “Processing Fly Ash Stabilized Hydrogen Titanate Nano-Sheets for Industrial Dye-Removal Application”, Journal of Hazardous Materials 2012, 229-230, 177-182) wherein, the removal of heavy metal-cations and organic synthetic-dyes from the aqueous solutions via the surface-adsorption process, involving the ion-exchange and electrostatic-attraction mechanisms operating in the dark-condition, using the hydrothermally processed nanotubes of semiconductor-oxides such as the hydrogen titanate (H2Ti3O7, HTN) and anatase-titania (TiO2, ATN) have been demonstrated. The HTN and ATN possess very high specific surface-area typically about 100-200 times that of as-received flyash particles. Hence, the adsorption-capacity of HTN and ATN for adsorbing organic synthetic-dyes and heavy metal-cations is extremely large. However, the major drawbacks of prior art-3 are as follows.                (9) HTN and ATN cannot be separated quickly from the treated aqueous solution via gravity settling.        (10) HTN and ATN are non-magnetic; hence, they cannot be separated from the treated aqueous solution using an external magnetic field.        
In view of the prior arts 1-3 and their limitations, it appears that there is a need for the development of novel composite materials which would exhibit higher capacities for surface-adsorbing the organic synthetic-dyes and heavy metal-cations and can be separated quickly from the treated aqueous solution via gravity-settling or using an external magnetic field. Flyash particles have lower dye-adsorption capacity but can be separated from the treated aqueous solution via the gravity settling. On the other hand, the nanotubes of semiconductor-oxides have higher dye-adsorption capacity but cannot be separated from the treated aqueous solution via the gravity settling. Hence, this suggests that a micro-nano composite material consisting of the nanotubes of semiconductor-oxides, such as the hydrothermally processed HTN and ATN, deposited on the surface of flyash particles can serve the purpose. Reference may be made to (S. Shukla, K. G. K. Waffler, M. R. Varma, M. T. Lajina, N. Harsha, C. P. Reshmi, “Magnetic Dye-Adsorbent Catalyst”, U.S. patent application Ser. No. 13/521,641 (Filed on 11 Jul. 2012), PCT Application No. PCT/IN2010/000198 (Filed on 29 Mar. 2010); L. Thazhe, A. Shereef, S. Shukla, R. Pattelath, M. R. Varma, K. G. Suresh, K. Patil, K. G. K. Warrier, “Magnetic Dye-Adsorbent Catalyst: Processing, Characterization, and Application”, Journal of American Ceramic Society 2010, 93(11), 3642-3650) wherein, the magnetic dye-adsorbent catalyst, consisting of the “core-shell” nanocomposite particles with the core of a magnetic ceramic particle and the shell of nanotubes of semiconductor-oxide, has been developed via the hydrothermal treatment of magnetic photocatalyst (processed via the Stober and sol-gel methods) followed by typical washing-cycles, to facilitate the quick settling of HTN and ATN using an external magnetic field. Reference may be made to (C. C. Sheng, L. T. Gui, C. X. Hua, L. L. Wu, L. Q. Cheng, XQing, N. Z. Wu, “Preparation and Magnetic Property of Multi-Walled Carbon Nanotube/α-Fe2O3 Composites”, Transactions of Nonferrous Metals Society of China, 2009, 19, 1567-1571; E. Santala, M. Kemell, M. Leskelä, M. Ritala, “The Preparation of Reusable Magnetic and Photocatalytic Composite Nanofibers by Electrospinning and Atomic Layer Deposition”, Nanotechnology 2009, 20, 035602; S. K Mohapatra, S. Banerjee, M. Misra, “Synthesis of Fe2O3/TiO2 Nanorod-Nanotube Arrays by Filling TiO2 Nanotubes with Fe”, Nanotechnology 2008, 19 315601, Fei Liu, Yinji Jin, Hanbin Liao, Li Cai, Meiping Tong, Yanglong Hou, “Facile Self-Assembly Synthesis of Titanate/Fe3O4 Nanocomposites for the Efficient Removal of Pb2+ from Aqueous Systems”, Journal of Physical Chemistry A, DOI: 10.1039/c2ta00099g) wherein, the magnetic nanocomposites having the morphology other than the “core-shell” morphology, consisting of magnetic nanoceramic particles deposited on the surface of TiO2 nanotubes, have also been processed via different techniques including the precipitation-calcination, electrospinning-atomic layer deposition, pulsed electrodeposition, and self-assembly process. However, the major drawbacks of prior art-4 are as follows.                (11) The combination of sol-gel and hydrothermal methods is not applicable to the flyash particles since SiO2, a major constituent of flyash particles, is soluble in a highly alkaline aqueous solution involved in the hydrothermal treatment.        (12) Innovative techniques for depositing the nanotubes of semiconductor-oxides on the surface of flyash are not known.        (13) Other techniques including the precipitation-calcination, electrospinning-atomic layer deposition, and pulsed electrodeposition are not suitable for depositing the nanotubes of semiconductor-oxides on the surface of flyash.        (14) The self-assembly process produces the magnetic nanocomposite with the magnetic nanoparticles uniformly dispersed on the surface of semiconductor-oxides nanotubes which reduces the potential sites on the surface of nanotubes required for the adsorption of dye molecules and the metal-cations from the aqueous solutions. Moreover, the self-assembly method also requires the use of an acid for obtaining the said morphology.        (15) Innovative techniques for attaching or anchoring the HTN or ATN to the surface of magnetic nanoparticles, typically at their short-edges (tube-openings), are not currently available. As a result, the said product cannot be synthesized using any of the existing processes.        
As a consequence, there is an urgent need to develop novel methods for depositing the nanotubes of semiconductor-oxides on the surface of flyash particles. Since the flyash particles are non-magnetic, they cannot be separated from an aqueous solution using an external magnetic field. Hence, it is also essential to demonstrate the deposition of the nanotubes of semiconductor-oxides on the surface of magnetic metal-oxide nanoparticles (instead of flyash) by attaching or anchoring them to the magnetic particle-surface using the same innovative mechanism which is employed in the case of flyash particles. Thus, the novel composite materials consisting of the nanotubes of semiconductor-oxides deposited on the surface of both the non-magnetic flyash and attached to (or anchored to) the surface of magnetic metal-oxide nanoparticles, via an innovative approach, would provide new ways of efficiently treating the aqueous solutions containing the harmful organic synthetic-dyes and heavy metal-cations. It would also provide new ways for the separation and recycling the flyash, without and with the surface-adsorbed metal-cations, as value-added products for the dye-removal application. As mentioned above, flyash (without and with the surface-adsorbed metal-cations), HTN, ATN, and magnetic composites can be used as dye-adsorbents. In order to recycle these dye-adsorbents as catalysts for the next-cycles of dye-adsorption, it is necessary to remove the previously-adsorbed dye from their surfaces. Reference may be made to (Z. Geng, Y. Lin, X. Yu, Q. Shen, L. Ma, Z. Li, N. Pan, X. Wang, “Highly Efficient Dye Adsorption and Removal: A Functional Hybrid of Reduced Graphene Oxide-Fe3O4 Nanoparticles as an Easily Regenerative Adsorbent”, Journal of Materials Chemistry 2012, 22, 3527-3535; M. Visa, L. Andronic, D. Lucaci, A. Duta, “Concurrent Dyes Adsorption and Photo-Degradation on Fly Ash Based Substrates”, Adsorption 2011, 17, 101-108; J.-T. Li, B. Bai, Y-L. Song, “Degradation of Acid Orange 3 in Aqueous Solution by Combination of Fly Ash/H2O2 and Ultrasound Irradiation”, Indian Journal of Chemical Technology 2010, 17, 198-203) wherein, annealing under the moderate conditions (at 400° C. for 1 h) for removing the previously adsorbed Rhodamine B dye from the surface of reduced graphene oxide-Fe3O4 composite has been reported. The mechanical mixture of flyash and TiO2 powders has been employed for the decomposition of organic synthetic-dye on the surface of flyash under the ultraviolet (UV)-radiation exposure. The recycling of flyash via the simultaneous dye-adsorption on its surface and dye-degradation using the combination of H2O2 and ultrasound-irradiation (Fenton-like reaction) has been reported. Reference may be made to (S. Shukla, K. G. K. Warrier, K. B. Babitha, “Methods for Decomposition of Organic Synthetic-Dyes using Semiconductor-Oxides Nanotubes via Dark-Catalysis”, PCT Application No. PCT/IN2013/000319 (Filed on 17 May 2013), Indian Patent Application 2555DEL2012 (Filed on 17 Aug. 2012)) wherein, the combination of hydrothermally processed HTN or ATN and H2O2 has been used to degrade the previously adsorbed organic synthetic-dye in an aqueous solution, typically in the dark-condition, without the use of external-irradiation and external power-source. In such case, the dye-decomposition is achieved through the generation and attack of both the free hydroxyl-radicals (OH−) and superoxide-ions (O2−) which are generated by the HTN and ATN in the presence of H2O2. Reference may be made to (M. S. Yalfani, S. Contreras, F. Medina, J. Sueiras, “Direct Generation of Hydrogen Peroxide from Formic Acid and O2 using Heterogeneous Pd/α-Al2O3 Catalysts”, Chemical Communications, 2008, 3885-3887) wherein, the Pd-deposited on alumina (Al2O3) substrate has been utilized to generate H2O2 in-situ using the formic acid (HCOOH) and dissolved oxygen (O2) which can be utilized for the decomposition of organic synthetic-dyes in an aqueous solution via the Fenton/Fenton-like reactions. However, the major drawbacks of prior-art 5 are as follows.                (16) Innovative techniques for removing the previously adsorbed-dye from the surface of various adsorbents mentioned above, for recycling them as catalysts, are unknown.        (17) The annealing treatment, conducted even under the moderate conditions, can destroy the nanotube-morphology and the phase structure;        (18) Flyash and TiO2 cannot be separated, after the dye-decomposition, from their mechanical mixture since both are non-magnetic; hence, the recycling of flyash is not possible from the mechanical mixture of flyash and TiO2 powders for reusing the former separately for the dye-adsorption process conducted in the dark-condition.        (19) The use of a magnetic photocatalyst, consisting of the “core-shell” nanocomposite particles with the core of a magnetic ceramic particle, an intermediate insulating layer of silica (SiO2) or an organic polymer, and the shell of nanocrystalline particles of semiconductor-oxide photocatalyst such as anatase-TiO2, has never been reported for the recycling of dye-adsorbents typically the flyash without and with the surface-modifications.        (20) The adsorbents such as the HTN and ATN cannot be recycled via the mechanically mixing and UV-exposure method involving the use of TiO2 photocatalyst since the nanocrystalline TiO2 particles have a tendency to get attached to the HTN and ATN via an ion-exchange mechanism (which has been disclosed here) which reduces the total number of potential-sites available for the dye-adsorption for a given amount of dye-adsorbent. The latter issue becomes severe with the increasing number of dye-adsorption cycles.        (21) The recycling of flyash particles using the combination of H2O2 (Fenton-like reactions) and ultrasound-irradiation is a costlier process.        (22) The efficient methods to recycle the flyash, in the dye-removal application, without the use of external-power source are not known.        
Hence, it is essential to develop simpler, easier, cost-effective, efficient, and innovative processes to remove or decompose the previously adsorbed-dye from the surface of dye-adsorbents, typically the flyash without and with the surface-modifications, to make their recycling possible in the dye-removal application.